1. Field of the Invention
The present invention relates to a nitride compound semiconductor light emitting device formed on a GaN substrate, and a method for producing the same.
2. Description of the Related Art
Conventionally, nitride compound semiconductor light emitting devices have been studied or utilized as light emitting devices or high power devices. In the case of light emitting devices, for example, light emitting devices covering a wide range of colors from blue to orange can be technically realized by using nitride compound semiconductors of various compositions. By taking advantage of such properties of nitride compound semiconductors, blue and green light emitting diodes (LEDs) have been realized in recent years. As for laser devices, blue-violet semiconductor laser devices are under development.
By using a nitride compound semiconductor, a light emitting device is typically produced as follows. A current injection layer having n-type properties is formed on a substrate, e.g., a mirror-polished sapphire (0001) substrate, upon which a nitride compound semiconductor is to be epitaxially grown. An active layer and a current injection layer containing an acceptor impurity are further formed thereupon. It is known that the use of a quantum well layer having a thickness of about 10 nm or less for an active layer results in a high emission intensity. The emission wavelength can be varied by adjusting the In (indium) component ratio in an InGaN active layer, for example. After the entire light emitting device structure has been formed, the device is subjected to a heat treatment in an N2 gas, whereby the acceptor is activated so as to impart p-type properties thereto. Thus, an LED or a laser device is completed.
In general, by doping an n-type nitride compound semiconductor crystal with Si using a SiH4 gas during a crystal growth process, for example, an electron density of 1018 cm−3 or more can be easily obtained. On the other hand, in order to obtain a hole density on the order of 1018 cm−3 with a p-type nitride compound semiconductor crystal, it is necessary to dope the p-type nitride compound semiconductor crystal with Mg using CP2Mg (Bis(cyclopentadienyl) Magnesium) or EtCP2Mg Bis(Ethylcyclopentadienyl) Magnesium) during a crystal growth process, and after the entire light emitting device structure including an active layer has been formed, subject the device to a heat treatment in an inert gas such as N2. A p-crystal having a high hole density “as grown” has not hitherto been obtained.
As used herein, a “high hole density” means a density of 1017 cm−3 or more. The expression “as grown” is used to describe a device which, after crystal growth has taken place, has not been subjected to a heat treatment, electron beam irradiation, etc. An “acceptor doping layer” means a layer which has been doped with an acceptor impurity, e.g., Mg.
The reason why an acceptor doping layer does not exhibit a p-type conductivity “as grown” under the conventional methods is that Mg atoms which are taken into the mother crystal have been inactivated by hydrogen. Specifically, a nitride compound semiconductor crystal which has been formed on a conventional sapphire substrate has a high concentration of defects and/or nitrogen vacancies due to a lattice mismatch as high as 13% with the sapphire substrate. Therefore, Mg atoms cannot be taken into the crystal by themselves, but rather are entrapped in an inactive state, i.e., Mg—H. Accordingly, in order to sever the Mg—H bonds so as to obtain active Mg atoms, it is necessary to apply thermal energy at a temperature on the order of several hundred ° C. in an inert gas atmosphere free of hydrogen after the light emitting device structure has been formed.
However, even after a heat treatment, which damages a thermally unstable active layer containing In, the resultant hole density would be between a latter half of the 1017 cm−3 order to the 1018 cm−3 order. To reduce the operation voltage in a light emitting device, it is necessary to reduce a contact resistance when a p-type electrode has been formed. Therefore, there is a desire for achieving an increased hole density in a p-type layer. In particular, a device which operates with a high current density, e.g., a laser device, is likely to be heated due to a high contact resistance, so that degradation may begin from an interface between the electrode and the p-type layer, leading to electrode destruction. In addition, excessive heating might cause deterioration associated with the mobility of or increase in dislocations within the light emitting device, resulting in a decrease in the emission intensity or fluctuation in the emission wavelength. Thus, the low hole density level in a p-type layer presently achievable under the conventional technique is detrimental to the emission characteristics and/or longevity of light emitting devices.
Moreover, a light emitting device formed on a conventional sapphire substrate not only suffers from the inactivation of Mg, but also receives unfavorable influences on an InGaN multiple quantum well active layer. As mentioned above, a light emitting device which includes an InGaN quantum well active layer formed on a sapphire substrate has a substantially incommensurate lattice constant with that of the sapphire substrate, and hence has a high concentration of nitrogen vacancies and/or threading dislocations, i.e., dislocations penetrating the device from the substrate interface to the device surface through the quantum well structure. In particular, a current which flows through a threading dislocation is a component which does not contribute to emission, and therefore increases the driving current density in the light emitting device, inducing heating within the light emitting device. Moreover, since a nitride compound semiconductor containing In is very unstable in terms of chemical-thermal equilibrium during a crystal growth process, a high concentration of dislocations are present. In the presence of such a high level of undulation in the underlying layer, each layer in the multiple quantum well structure formed thereon will have a non-uniform thickness.
As a method for solving the problems of dislocations and nitrogen vacancies, Japanese Laid-Open Patent Publication No. 9-23026 discloses a technique of performing a two-part growth involving a buffer layer, where an angle between a sapphire substrate and the (0001) plane is maintained equal to or less than 5°, thereby reducing dislocations and improving emission characteristics. There is also disclosed a technique of, after growing a single quantum well active layer of InGaN, interrupting the growth or observing a wait period for 60 minutes or less to obtain a light emitting device having a uniform emission state and high yield. Japanese Laid-Open Patent Publication No. 10-126006 discloses that a quantum well laser device having a low threshold current density can be formed by forming a well layer to become an active layer in a three-well quantum well structure, observing a wait period for 2 to 10 seconds, and then forming semiconductor layers.
However, in all of the aforementioned conventional techniques, a heat treatment for imparting p-type properties is required after forming a light emitting device structure. Due to an insufficient carrier density, a sufficiently low p-type contact resistance has not been realized. In addition, the problems associated with a heat treatment for imparting p-type properties, e.g., a damaged active layer, non-uniform composition of In-containing layers, non-uniform layer thicknesses, deterioration in crystal quality, etc., have not been solved. Therefore, it is difficult with conventional techniques to produce a high-efficiency LED or a low-threshold semiconductor laser device which requires a reduced operation voltage and/or current. Thus, there is a need for a technique of producing light emitting devices having improved characteristics.